1. Field of the Invention
The present invention relates to a GaN single-crystal substrate, a nitride type semiconductor epitaxial substrate, a nitride type semiconductor device, and methods of making them for use in light-emitting devices and the like.
2. Related Background Art
Attention has recently been given to light-emitting devices using nitride type compound semiconductors, since they can emit light having a short wavelength in a region from ultraviolet to blue-green. These devices such as light-emitting diodes and laser diodes are prospective as illumination and display apparatus, or light sources for next-generation DVD. As a substrate for use in these light-emitting devices, a GaN single-crystal substrate having a lattice constant identical to that of a GaN layer, which is a major nitride type semiconductor layer, is preferably employed. However, it has conventionally been considered difficult to make a GaN single-crystal substrate.
Therefore, sapphire substrates, which have a lattice constant approximating that of GaN while being chemically stable, have normally been in use. OMVPE is usually used as a method of epitaxially growing a GaN substrate on such a sapphire substrate. In the OMVPE, after the surface of the sapphire substrate is cleaned while the substrate temperature is held at about 1050° C. in an H2 gas atmosphere, a buffer layer of GaN or AlN is grown at a substrate temperature of about 450 to 600° C., and then a GaN layer is grown at a high temperature of at least 1000° C.
However, the use of a sapphire substrate is problematic in the following points: First, the lattice constant of the sapphire substrate is close but not identical to that of the GaN layer, whereby a number of defects such as dislocations caused by lattice mismatching are introduced at the interface between the sapphire substrate and GaN layer. These defects extend in the growth direction, so as to appear as a number of through defects on the epitaxial layer surface, and remarkably deteriorate characteristics and lives of light-emitting devices such as laser diodes. Also, since the thermal expansion coefficient of the sapphire substrate and that of the GaN layer greatly differ from each other, the eptaxially grown substrate may warp greatly. Further, since the sapphire substrate has no cleavage property, it is very hard to make a laser diode using a cleavage plane as a reflecting surface.
In view of such circumstances, a single-crystal GaN substrate suitable for forming a nitride type compound semiconductor layer has been realized (International Application Laid-Open No. WO99/23693). In this method, a mask having a striped or circular form is formed on a GaAs substrate, a GaN layer is grown thereon in a vapor phase, and then the GaAs substrate is removed therefrom, whereby a GaN substrate can be obtained. Also, this method can mass-produce GaN substrates by further growing a GaN layer on the GaN substrate so as to prepare an ingot and then cutting out GaN substrates from the ingot. Namely, this new method has made it possible to mass-produce GaN single-crystal substrates.